Raman spectroscopy is a proven technology in bio-medical, chemical, industrial and other sensing applications. However, significant problems exist for implementing this technique, such as detector sensitivity, processing speed, simultaneous multi-component analysis of a single sample, environmental ruggedness, and cost. In order to obtain Raman spectra from a sample, a high intensity optical source is needed (typically a laser) to pump the inelastic Raman scattering process within the material, be it a gas, a liquid, or a solid. As a result, the material scatters radiation, in all directions, at different frequencies. The component with frequency equal to that of the pump laser corresponds to Rayleigh scattering, and the component with frequency shifted lower than that of the pump laser is called Stokes radiation, a portion of which corresponds to Raman scattering. The main feature of Raman scattering is that it occurs regardless of the wavelength of the pumping optical source, while keeping the frequency shift between Stokes and pump radiation fixed. The Stokes radiation shift and intensity are dependent upon the material. Typically, Stokes Raman shifts are in the order of a few to tens of tera-Hertz (THz), and their intensity is 4 to 5 orders of magnitude lower than the Rayleigh scattered light. In order to discriminate and measure accurately the Raman scattered radiation from the Rayleigh radiation, a blocking filter for the Rayleigh frequency needs to be used in all Raman measurement systems. Fortunately, the typical Raman Stokes shift is large enough to allow for current state-of-the-art filters to block the Rayleigh radiation while marginally affecting the Raman Stokes radiation.
Time-resolved Raman spectroscopy techniques have been used for years. Detection and analysis of the signal in these systems is typically difficult and expensive. Commercial Raman spectrometers are:
1) too slow for many practical applications, with signal processing time of a few seconds or more. Real-time process monitoring is impossible, as are many medical and in-vivo applications;
2) typically limited to measuring no more than two or three components within a given sample, at a time, due to high spectral overlap between different analytes;
3) physically sensitive to the environment such as movement, vibration, and temperature changes, in their performance; and
4) not optically sensitive for many applications such as detecting weak markers in biological samples or weak returns and noisy signals from long-range sensing applications.
Techniques for processing multiple components, in the order of 20 to 100, with a 1 to 10 second typical collection for each, require an excessive amount of time to complete a full sample analysis. Weak signals from noisy environments result in the loss of important spectral information in many cases. Field applications in harsh environments are also off limits for currently available Raman systems.
The most popular types of spectrometers in use today are Fourier-Transform type devices. Fourier Transform Infra-red (FTIR) and Fourier Transform Raman (FTR) spectrometers employ a motor to create a linear displacement of sensitive optical elements in the detection process. This technique has serious operational and environmental limitations, since alignment must be maintained as optical parts are being moved, and also time-calibration is necessarily complex since non-uniform linear motion is involved.
Accordingly, there is a need for simpler, environmentally insensitive Raman spectrometers capable of determining multiple components in a sample within a very short time.